1. Field of the Invention
The present invention relates to an insulated gate bipolar transistor (hereinafter referred to as "IGBT") utilized as a power switching element for high voltage and heavy current. More particularly, the present invention relates to the IGBT with a guard-ring structure.
2. Description of the Related Art
An IGBT element, while having a configuration similar to a power MOSFET, has a pn junction disposed in a drain region. Therefore, the conductivity of a highly resistive drain layer is modulated during operation so that the IGBT element can achieve both a high withstand voltage and a low ON resistance, which are impossible for the power MOSFET to achieve. A guard ring structure is used as a high voltage withstand means in the periphery of an IGBT cell region. The guard ring structure surrounds the IGBT cell region and helps improve withstanding voltage because distribution of the electrical field becomes a step-like configuration. FIG. 3 shows a main cross-sectional view of a conventional IGBT element with a guard ring structure.
In FIG. 3, when a surge voltage is applied between drain electrode 1 and source electrode 9, pn junction 2 defined between p base layer 7 and n.sup.- drain layer 3 is brought into a reverse biased condition, so that a depletion layer (not shown) propagates in n.sup.- drain layer 3. In the middle portion of the cell region A where p base layer 7 and n.sup.+ source layer 8 are plurally disposed, the depletion layers not shown) extend from the adjacent p base layers 7 toward n.sup.- drain layer 3, which is situated between the adjacent p base layers 7, and connect with each other so that the electrical field relaxes. Accordingly, the electric field in the middle portion of the cell region A is settled by the maximum value E.sub.A at a flat portion of pn junction 2, i.e., the bottom portion of p base layer 7.
On the other hand, in the peripheral portion of the cell region A, because repetition of p base layers 7 stops, the above-mentioned effect of relaxing the electric field, which is obtained in the middle portion of the cell region A, is not obtained in the peripheral portion of the cell region A. Therefore, the electric field in the peripheral portion of the cell region A is settled by the maximum value E.sub.B at the corner portion of the peripheral p base layer or at a surface of n.sup.- drain layer 3 proximate to the peripheral p base layer. Generally, E.sub.B is larger than E.sub.A. That is to say, the withstand voltage in the peripheral portion of the cell region A is smaller than that in the middle portion of the cell region A. Thus, the withstand voltage as the IGBT element is determined by this smaller withstand voltage in the peripheral portion of the cell region A.
To increase the withstand voltage in the peripheral portion of the cell region A, at region B, which is outside the circumferential portion of the cell region A, a plurality of p layers 6', 6 are disposed known as a guard ring 6. The guard ring structure is generally provided so as to reduce the maximum electrical field intensity E.sub.B to E.sub.A by disposing one or more p islands at the surface of n.sup.- drain layer 3 in region B. In addition to guard ring 6, a metal film 18, or a field plate, may be provided on n.sup.- drain layer 3. The metal film 18 making up field plate contacts guard ring 6 and spreads on a insulating film formed on n.sup.- drain layer 3. FIG. 3 shows the situation where field plate 18 is also included.
In this arrangement, when the surge voltage is applied to drain electrode 1 so that the maximum electrical field intensity E.sub.B in the guard ring region B reaches a critical electrical field intensity which causes an avalanche breakdown, a great number of electron-hole pairs is generated, one carrier flows into source electrode 9, and another carrier flows into p.sup.+ drain layer 4. The current which flows in the IGBT element, at this time, partially concentrates in the guard ring region B. Further, as the carrier which flowed into p.sup.+ drain layer 4 allows for a new injection of minority carriers into n.sup.- drain layer 3, a larger current flows, and the current density in the guard ring region B is larger, so that breakdown occurs due to the localized heavy current and the concentrated electric field. This is why large withstand voltages against an avalanche breakdown cannot be obtained. That is, when a high surge voltage is applied to drain electrode 1 of the IGBT element, even though the guard ring structure is provided as shown in FIG. 3, an avalanche breakdown occurs in the guard ring region B and surge energy is locally applied, which thereby leads to destruction of the IGBT element. The withstand voltage against the avalanche breakdown is improved by deepening the depth of the diffused layer making up guard ring 6, which is disposed in region B, or by increasing the number of diffused layers making up the guard ring 6.
However, because deepening the depth of the diffused layer making up guard ring 6 of the IGBT element makes the lateral diffusion length increase, the guard ring region area needs to be expanded. Moreover, as diffused layers making up guard ring 6 in region B are generally formed at the same time with deep p well layers, which constitute p base layer 7 in the cell region A, so as not to increase the number of photolithographic processes, problems are caused, such as the width of the deep p well layer in the cell region A increases and the chip area is also increased. Further, multiplying the number of diffused layers making up guard ring 6 disposed in region B increases the area of guard ring region and leads to expansion of chip area.